While modern crocodilians are all semi-aquatic, their Mesozoic ancestors (known as neosuchians) started off fully terrestrial, only really moving into their familiar water-based ecological niches around the mid-Jurassic when the dinosaurs were dominating on land.
But on multiple occasions members of the neosuchian croc lineage independently went back to fully terrestrial habits, and Tarsomordeo winkleri here is one of the most recently discovered examples.
Living about 113 million years ago in the Early Cretaceous of central Texas, USA, Tarsomordeo was surprisingly small, only about 60cm long (2′) – the size of an average cat. Its tiny size even ended up inspiring its name, which translates to “ankle biter”.
It had long slender limbs held in an upright posture, suggesting it was a swift and agile runner capable of chasing after fast-moving prey. Since it lived in a semi-arid environment that seems to have been a major nesting site for the herbivorous Convolosaurus, their hatchlings probably also made up a large part of its diet during the breeding season.
The placoderms are most famous for some of the biggest members of the group such as the giant blade-jawed Dunkleosteus. But these ancient armored fish were actually incredibly diverse in their time, occupying many different ecological niches and developing a wide range of body shapes.
Perhaps one of the most unusual was Brindabellaspis stensioi from the Early Devonian of New South Wales, Australia. Living around 405 million years ago in a tropical reef ecosystem, this early placoderm was quite small, only about 45cm long (1′6″), and it was recently revealed to have had an especially weird head.
Its skull was flattened with its eyes facing upwards on top, its nostrils came out of the corners of its eye sockets, and its jaws were positioned very far forward. It also had a long flat snout packed full of sensory nerves, sort of like the bill of the modern platypus but using a modified form of the pressure-sensing lateral line system instead of electroreception.
It was probably some sort of bottom-feeder, using its bill to feel around on the seafloor for small prey – and there may even have been a longer and wider soft tissue extension to its sensitive snout, giving it even more of a duck-like shape.
For around 50 years some very unusual dinosaur tracks have been found in ancient desert sediments in South America: strange footprints showing the impression of only a single toe, a walking style never before seen in any reptiles.
And recently a fossil of what might be the track maker has actually been found.
Named Vespersaurus paranaensis, this new species lived during the Late Cretaceous of Brazil (~90 mya) and was a member of the noasaurid family of theropods, closely related to the weird-jawed Masiakasaurus from Madagascar.
Measuring about 1.5m long (~5′), Vespersaurus was fairly lightly built with legs proportioned for running – and its feet were absolutely unique. Although it had the standard three main toes of a theropod, it bore its weight entirely on the middle toe and held the other digits off the ground. The two raised toes on each foot also had large knife-like claws which may have been used during hunting, vaguely similar to the sickle claws on the feet of dromaeosaurs. But unlike dromaeosaurs these claws weren’t highly curved or pointed, suggesting Vespersaurus used more of a scratching and slashing technique rather than the raptors’ puncture-and-restraint strategy.
Much like ancient horses, it may have developed its single-toed stance as an adaptation for more efficient fast running, possibly to avoid larger predators or to chase down small fast-moving prey like hopping desert mammals.
The known one-toed fossil footprints are actually slightly older than the Vespersaurus fossil, and similar tracks in Argentina have been found dating back to the Late Jurassic (~150mya), so there may have been a long lineage of “one-toed” desert-dwelling noasaurids in South America that haven’t been found yet.
The exact evolutionary relationships between the earliest amphibians and amniotes is rather murky, and the recently-discovered Diabloroter bolti here is a member of a group in the middle of this uncertain classification.
It was part of a lineage known as the recumbirostrans – small burrowing aquatic salamander-like creatures, many of which had elongated bodies and short tails. Although traditionally considered to be lepospondyl “amphibians”, more recent studies have suggested that these animals might instead have been very early true amniotes related to early reptiles.
Measuring only about 6cm long (2.4″), Diabloroter is known from a single fossil from Illinois, USA, dating to the Late Carboniferous about 309-307 million years ago. Its anatomy indicates it was probably a herbivore – making it one of the earliest known plant-eating tetrapods – with teeth adapted for scraping at algae-covered surfaces and a rather rotund body that would have housed a large gut region.
It also had fairly well-developed limbs, which were probably used for burrowing like many of its close recumbirostran relatives, but may also suggest it spent a lot of time walking around on land.
The distinctive armored ankylosaurs and stegosaurs were very closely related to each other, and were part of a group of dinosaurs known as the thyreophorans.
One of the earliest known members of this lineage was Scutellosaurus lawleri. Living in Arizona during the Early Jurassic, about 196-183 million years ago, it was a small lightly-built bipedal herbivore, only about 1.2m long (3′11″) – with over half that length being just its unusually long tail.
Its body was covered in rows of hundreds of small bony osteoderms, helping to protect it against larger predators like Dilophosaurus. And this was obviously an evolutionary strategy that worked very well for Scutellosaurus and other early thyreophorans, because within about 20 million years they’d given rise to the first true ankylosaurs and stegosaurs – with the tank-like ankylosaurs being especially successful, spreading to every continent and lasting all the way up until the end-Cretaceous mass extinction.